The present invention relates to a method of forming cermet layers, and particularly, to such a method wherein cermet resistors having low temperature coefficients of resistance, high sheet resistance and good thermal stability can be prepared.
Cermet layers or films have found extensive use as resistors in microelectronic devices, integrated semi-conductor circuits and in hybrid thick film circuits. Conventional methods of forming the cermet layers include co-evaporating or co-sputtering metals and ceramics, painting or silk screening mixtures of metals and ceramic particles onto suitable substrate surfaces, and by chemical vapor deposition of a cermet layer in a reactive oxiding atmosphere.
Hybrid circuits usually include conventional semiconductor chips containing transistors and diodes. The chips are mounted on an insulating substrate which also contains passive components such as resistors, capacitors and inductors in any one of several different forms. For example, the passive components may be conventional, discrete, miniature-sized units mounted separately on the substrate and connected together with printed wiring or soldered wires. Or, more desirably, the passive circuit components may be a deposited film.
For many applications, it is desirable to obtain a film having a high sheet resistance and a low temperature coefficient of resistivity (TCR). For example, U.S. Pat. No. 3,669,737 issued June 13, 1972, discloses a thin film electrical resistance element having a sheet resistance of from about 50 ohms/square to 5000 ohms/square. The electrical resistance element of the previously mentioned U.S. patent is a layer consisting of a mixture of tungsten and tungsten oxide deposited by the pyrolytic decomposition of tungsten hexacarbonyl. By the addition of varying amounts of oxygen, the sheet resistance of the deposited film can be selected within the previously mentioned range. It would therefore be desirable to develop a cermet layer or region having a relatively high sheet resistance, i.e., greater than 5000 ohms/square, and a low temperature coefficient of resistivity.